The present invention relates to combustion methods and burners producing very low levels of pollutants such as CO and NO.sub.2.
The invention is particularly applicable to gas-fired radiant burners having a combustion surface at or near which a gaseous fuel, such as natural gas, is combusted. The burners have been developed primarily for use in respect to gas burner space heating devices and will be described with reference to this particular use. However, it should be appreciated that the invention is not limited to this particular field of use and is equally applicable to other types of gas burners as well as other burners that use a variety of different fuels.
Gas-fired burners are widely used commercially and in the domestic environment for heating including space heating for temperature conditioning interior space. Such burners are generally preferred over electricity due to their low cost and efficiency as well as their general flexibility. However, burners of all kinds are now known to be a source of indoor pollution especially in the amounts of oxides of nitrogen (NO.sub.x) formed.
NO.sub.x is a term used to describe the combined oxides of nitrogen and in particular NO, N.sub.z O and NO.sub.2. For example, NO and N.sub.2 O are a concern in the outdoor environment, in particular with relation to acid rain, O-zone and photochemical smog. However, NO.sub.2, is of more concern to medical authorities due to its affect on lung function. Medical research during the 1980's has suggested that lung function will be affected by much lower levels of NO.sub.2 than was previously thought. This has led to severe restrictions on the acceptable emission levels of NO.sub.x with particular emphasis on the emission of NO.sub.2.
Radiant burners which include combustion surfaces, are generally preferred over blue flame burners which tend to produce NO.sub.2 in the levels in the order of 15 to 30 ng/Joule and as such are not considered to have potential for the reduction of NO.sub.x. For this reason NO.sub.x gas burner research has centered primarily around surface combustion burners of different forms.
In the last twenty years, research into the production of gas burners having lower NO.sub.x emission levels has concentrated on the use of excess air, alone or in combination with the incorporation of second stage burning. As a result, a number of these burners have become very complex in both design and operation procedures.
For example, up until recently, the most successful burner design had been based on using pressurized pre-mixed air/fuel mixtures burned in a variety of metallic surface configurations, ceramic surfaces or after-burners. All such burners relied on high excess air and high combustion load. The associated requirements of pressurizing systems, after-burners and high combustion loads resulted in gas burners that were often bulky, complicated and inflexible in their operation as well as costly.
Furthermore, while some of these burners succeeded in reducing NO.sub.x emission levels relative to the older types of burners, it still appeared impossible to approach the desired target levels.
However, a recent development in surface combustion burners as taught in Australian Patent Application No. 64743/90 and corresponding U.S. patent application Ser. No. 598,021, filed Oct. 16, 1990, owned by the assignee of this application and incorporated herein by reference, has resulted in the production of a surface combustion gas burner having unexpectedly low emission levels of NO.sub.x including both NO and NO.sub.2. This burner will hereinafter be referred to as the "Bowin" burner.
The present invention arises from further development work performed initially on the low NO.sub.x burner described above. This work has shown that further substantial reductions in the emission levels of NO.sub.2 can be achieved and in some cases all measurable traces of NO.sub.2 can be completely eliminated. Through a series of experiments it was shown that most burners can be modified to inhibit the production of NO.sub.2 in a controlled manner previously unknown within the industry.
The test work performed during development of the previously described Bowin burner indicated that the production of oxides of nitrogen can be controlled by, among other things, controlling the flame temperature within certain limits to inhibit the formation of NO. It is believed that NO is the precursor of NO.sub.2 which forms by subsequent oxidation of the NO. Accordingly, by reducing the formation of NO and suppressing the combustion temperature, it was also possible to also limit the formation of NO.sub.2 to very low levels.
However, up until now it has not been possible to eliminate the production of NO.sub.2, completely, since at least a portion of the NO.sub.2 is formed from NO outside the burner at a stage over which there is little or no control.
The present invention provides a mechanism to prevent the suspected conversion of NO to NO.sub.2 by a simple technique that can be adapted to apply to most types of burners.
The radiant burners of particular interest herein are non-powered burners in that they do not include a powered fan or blower assisted supply of combustion air. A pressurized source of fuel is used to aspirate the required combustion air at levels in excess of stoichiometric for delivery to the combustion surface. Heretofore, surrounding or ambient air was freely available to the combustion process occurring at or near the combustion surface. The ambient air is typically at a relatively cooler temperature as compared with the combustion gases.
Accordingly, it is an object of the present invention to provide non-powered radiant burner apparatus and methods of combustion so as to overcome or substantially ameliorate the foregoing disadvantages of the existing prior art burners and combustion techniques. It is a further object of the present invention to enable production and operation of pertinent burner types with reduced pollutants so that emissions of CO and NO.sub.2 are substantially reduced and/or eliminated.